After Richard (Leonardo DiCaprio) enjoys some pleasurable time on the island, which also saw Françoise (Virginie Ledoyen) leave Etienne (Guillaume Canet) for him, his and the beach community's lives turn sour. While out with the community leader Sal (Tilda Swinton) on the mainland getting some things for the community, the tourists he met earlier and gave a copy of the map to the island to, Sammy (Jerry Swindall) and Zeph (Peter Youngblood Hills) along with some girls they recently met up with, Hilda (Saskia Mulder) and Eva (Simone Huber) come up and call him "the man with the map' in front of Sal. Richard tells her he told them where he was going and showed them the map, but lied to her again when he said he didn't copy it. He and Sal also have sex before returning.
Some time after their return, Sten (Magnus Lindgren) and Christo (Staffan Kihlbom) are attacked by a great white shark (possibly getting revenge for Richard killing its baby), with Sten being killed and Christo suffering a badly wounded left leg. After refusing to leave to see a doctor, Sal objecting to a doctor coming to the island to treat him, and his cries of pain spoiling everyone's fun, Christo is exiled into the woods, cared for only by Etienne. Some time after that, an angry Sal brings Richard up on a hill to show him Sammy, Zeph, Hilda & Eva playing on the island directly across from them, carrying the copy of the map. She tells him to keep watch of them daily until they came, and get the map from them when they arrived and turn them away. Later on in the evening, Françoise comes to him and informs him that Sal told everyone about him sleeping with her, and because of it she was dumping him.
While Richard waited for Sammy & company to arrive, he found life on the hill to be a big improvement, based on the abundance of activities he saw to keep himself occupied and no responsibilities bogging him down. He soon saw no reason to return to the community. However, his increasing self-imposed isolation in the woods starts reducing him to a more primal state, and the beach community begins questioning his sanity. He also messes around with the marijuana farmers who shared the island, stealing from them and such, and he starts hallucinating about Daffy (Robert Carlyle), the man who gave him the map to the island before killing himself, and sees him as his ally and guide.
When Sammy and the others arrive at the island, the near-primal Richard follows them to the marijuana field to get the map. The farmers spot the four, and Sammy, Zeph, and Hilda are gunned down. Eva, the map holder, runs away, but Richard soon intercepts her. A farmer shoots and kills her, and blood sprays on Richard's face. This snaps him back to reality, and after the farmer who shot Eva spots him, he makes a frightened, hasty retreat back to the beach community. He finds Françoise, and together they go see Etienne, still caring for Christo who is now dying of gangrene. Richard tells them about what the farmers did, and that they had to leave now because he didn't know what else they would do. After telling them to wait for him at the boat Sal used to get to and from the island, he kills Christo to end his suffering. However, the marijuana farmers arrive as he is doing so, holding everyone at gunpoint in the main hut, and Richard, Francoise & Etienne are captured and brought there.
The senior farmer (Abhijati Jusakul) orders everyone to leave the island forever, but Sal refuses to do so, saying it was their home, too, and tells the farmer that Richard copied the map. He takes his handgun, loads it with a single bullet, and tells Sal that if they all want to stay, she would have to shoot Richard. As he stares down the barrel, Richard tells Sal that she can't do it, because it will not be like Christo rotting in the woods where no one could see or Sammy & company getting killed: This time everyone would have to watch and see what it takes to keep this paradise a secret. Sal pulls the trigger, but the gun is empty. The farmer pocketed the bullet. Everyone leaves the island and goes back to wherever it was they called home, except for Sal, because she believed in it all too much to ever change.
Some time later, Richard is shown back in America at an Internet cafe, and after accessing his e-mail account he finds Françoise sent him the picture she took of him and the beach community. In a voiceover, Richard says he still believes in paradise, but it isn't where you go, it's how you feel in a moment in your life when you're a part of something.
*I tried to interpret what was taking place in the last 40 minutes of the film (when Richard slides into madness) as best as I could. The film is terribly confusing during that stretch of time.
Thanks Tornado Dragon!